theres quite a few ways to do it.
create two layers
place the image on the bottom layer.
on the top layer create a gradient with the tool under the paint bucket tool, not under it as in vertical but the alternative tool using the small arrow in the bucket box tool panel.
(hold the left mouse and choose the gradient tool.)
left click and drag in your image to apply the gradient.
you could also use an airbrush for a blob or a shape if you wanted in this part.
you could also stroke a path with an airbrsuh at this point also.
the real trick to this is the next part by changing the top layers properties.
on the top layer change its usage to "multiply" instead of "normal" in the layer properties.
you could also do it using compound layer masks but the above technique is just one way that can make it happen.
If no-one gives your an answer to a question you have asked, consider:- Is your question clear.- Did you ask nicely.- Are you showing any effort to solve the problem yourself